The invention relates to a charged particle beam spectrometer.
Classical electrodynamics show that a charged particle moving with a velocity through a magnetic field oriented at right angles to the direction of a component of the particle's velocity will be angularly deflected by an amount dependent on the particle's mass, charge and velocity, as well as the strength of the magnetic field. If the charged particles in a beam all have the same mass and charge and all experience the same magnetic field, any differences in bend angle may be attributed to a difference in velocity (kinetic energy) of the particles. In such an arrangement, the greater the particle's velocity, the less its bend angle will be. Examples of equally charged particle beams include electron beams, proton beams, and ion beams.
Most electron energy spectrometers currently in use utilize a bending magnet to achieve primary separation of the various particle energy components and then direct the angularly separated beam to a beam detector. Measurement of the beams current (the number of charges passing a point in a second) in a conventional charged particle beam spectrometer is typically accomplished in one of two ways; foil-light emissions or Faraday cups.
In spectrometers utilizing foil-light emissions for beam current detection, a thin foil is placed transverse to the beam path at some point downstream of the magnet. As the particles intersect the foil, light is created from the particle collisions with the foil atoms. The amount of light created is a function of the number of particles involved in the collisions, and thus the beam current can be inferred from the light intensity profile along the foil. An advantage of this method is that it provides a continuous and instantaneous energy spectrum of the beam. That is, the divergence of the beam envelope in the momentum-dispersed direction defines the beam's entire energy spectrum, and as long as the foil is continuous where it interacts with the beam, the energy spectrum displayed by the foil will also be continuous. A disadvantage of this method is that beam current is inferred from foil light emissions only, and since the physics of these interactions can be quite complex, the values derived can be in error.
Another way commonly used for determining the current of each spectral component of the beam is by using a Faraday cup. Faraday cups use a conductor as a charge collector. Charged particles are directed to the charge collector, which captures the charged particles. The number of charged particles captured by the charge collector is measured with respect to time by a Faraday cup. A further explanation of Faraday cups is given by D. Pellinen in "A High Current, Subnanosecond Response Faraday Cup," in The Review of Scientific Instruments, Vol. 41, Number 9, pp 1347-1348, incorporated by reference. While the Faraday cup has the advantage of providing very accurate current readings, in order to give energy information it must be moved across the beam path in the energy-dispersed direction, since the Faraday cup can measure the current at only one location at a time. This will yield very accurate energy data, but does not allow an instantaneous reading of the beam's current and energy distribution. Designs for arrays of Faraday Cups either do not withstand prolonged use, or require extensive shielding and collimation prohibiting the close placement of sensing regions of the Faraday cups. A further description of problems with Faraday Cups is described by T.P. Starke in "High Frequency Faraday Cup Array," in The Review Of Scientific Instruments, Volume 51, Number 11, pp 1473-1477. It should be noted that Faraday Cups are not Faraday probes described in the invention below.
In addition to the above limitations, because both Faraday cups and foils intersect the beam, both methods are limited to beams of relatively low currents. Higher current beams would thermally or structurally damage these detectors. The spectrometers described above have been adequate for many charged particle beam applications which require the analysis of essentially monoenergetic beams (energy variations of less than 5%), and frequently the currents involved are not high enough to cause thermal or structural damage to the intersecting medium.
Spectral analysis of high power charged particle beams or charged particle beams having a broad energy spectrum with energy variations of greater than 5%, such as the charged particle beams produced by high-power Free Electron Laser amplifiers require a particle beam diagnostic more robust than the existing detectors.
Free Electron Lasers utilize an undulating relativistic electron beam to amplify a laser beam. Because the kinetic energy of the electrons is converted into photons, the light amplification increases with beam current and energy. This also means that as the beam exits the undulator it can have widely-varying energy components, which depend on the efficiency of the energy conversion process.